The present invention relates to an improvement of a presser or anvil in an ultrasonic welding apparatus, in which workpieces of a thermoplastic synthetic resin overlapped one above the other and held in forced contact between a working horn having a working surface which is to be imparted ultrasonic oscillation and a presser pressably disposed toward the working surface, for welding the workpieces by frictional heat produced there.
The presser of this kind has traditionally been made of iron or other metallic materials. Therefore, several disadvantages took place, when a pair of workpieces, for example, of thermoplastic synthetic resin, which are overlapped one above the other at one edge of each piece by being folded back, having thereby a raised or thick portion are seam welded using a cover film covering the thick portion and a thermo-adhesive film put under the cover film with the aid of frictional heat produced there. Principal factor to those disadvantages was the fact that the presser was made of a solid matter, which was liable to make the pressing force produced between the presser and the working horn uniform. If the pressing force is adjusted to suit a better welding condition at the thick portion the neighboring ordinary level portions are not welded satisfactorily. On the contrary, an increase of the pressing force at the presser with the object of improved welding effects at the neighboring ordinary level portions often causes a breakage or an unseemly appearance of the cover film at the thick portion.